Previous electron beam inspection systems generated an electron beam by a Schottky thermal field electron emitter. Surrounding the emitter was a suppressor electrode, which suppressed unwanted thermal electrons from the shank of the emitter from getting into the primary beam. A voltage on the extractor electrode determined the electric field at the apex of the tip. The field at the tip determined the amount of current that could be extracted from the tip. A second electrode or anode after the extractor was typically used to accelerate the beam to the desired energy. The current distribution from the emitter extended to several degrees. In an electron beam inspection system, small source acceptance angles of less than one degree may be required for high resolution. In a scanning electron microscope (SEM), a spray aperture is typically used to block unused electrons. A second aperture farther down the column can act as a pupil aperture, which sets a numerical aperture (NA) of the objective lens. Such a design still has problems with Coulomb effects.
Therefore, what is needed is an improved electron beam system.